I've found that imaging is mostly a factor of proper setup, proper room treatment and overall room acoustics. Most speakers will image if the above are addressed correctly. Yes, the speaker plays a part after that, but until you have those other 3 set right you can't really compare one speaker vs another to say which is "best".
Having said that, speakers that use concentric drivers or simulate point sources (like MTM's) tend to do a better job IMO.
Enjoy,
Bob
